The letters “zip” in “zip code” stand for Zone Improvement Plan, and zip codes were first implemented in 1963. A zip code identifies a specific geographic delivery area. Before zip codes were introduced, two-digit “zones” were used, and those zones represented a place in a city. The post office sorted mail first by state, then by city, then by zone, then by street address. With the coming of zip codes in 1963, three digits were added in front of the two-digit zones. Postal employees no longer needed to read the state and town when they had the five-digit zip codes. Increasingly, the postal service moved to mechanized reading and sorting. Often, mail in the United States now includes a bar code which contains zip code information to aid in this mechanized process.
In 1983, the “zip plus four” concept was introduced. The basic five-digit zip code remains, but a very specific four-digit number is added, again to help with mechanized reading and sorting. Sometimes the four digits stand for a city block, a specific building, or even—in a high rise apartment or office—a few floors. The average American does not use the “plus four” zip code, but it is required for bulk mailings. Use of the 4-digit add-on helps the postal service route mail more efficiently and accurately because it reduces handling, and significantly decreases the potential for human error and possibility of misdirected delivery; it also leads to better control over USPS costs and, in turn, postage rate stability. zip+4 is intended for use primarily by business mailers who prepare their mail with typewritten, machine-printed, or computerized addressing formats that can be read by the Postal Service's automated scanners during processing. The postal service has a zip+4 look-up system on its Internet server, in order to avail the public of a simple means of obtaining zip+4 code information.
The zip code is often translated into a barcode called POSTNET, that is printed on the mailpiece, to make it easier for automated machines to sort the mail. Unlike most barcode symbologies, POSTNET uses long and short bars, rather than thin and thick bars. The barcode can be printed by the person who sends the mail, or the post office can affix the barcode when the postal service receives the mail piece. If the postal service affixes the barcode, the destination address is determined using either optical character recognition (OCR), or a human reads the address if absolutely necessary.
People who send bulk mail can get a discount on postage if they have pre-printed the barcode themselves. An additional two digits are usually used to indicate the exact “delivery point” (DP), so that every single mailable point in the country should have its own 11-digit number. These two digits are usually the last two of the street address or box number, though non-numeric points with names or letters are assigned DP numbers by the local post office. A last digit can be used as a check digit, obtained by adding up the 5-, 9-, or 11-digits, and then subtracting the last digit of that result from 10 (e.g. the check digit for 10001-0001 00 would be 7). The sender needs only to type something like “100010001007” using POSTNET to create the code for printing.
When the owner of a mailbox or other mail delivery point wants to leave specific instructions for a mail delivery, a cumbersome process is usually needed. The owner must go to the post office and fill out forms, which then must be read and entered into a computer. Due to the cumbersome nature of this process, only a limited number of the most essential services are available in this way, including a change of address. Some of those services would be even much more cumbersome than the change of address process, because it can be difficult for a busy delivery person to be adequately informed that a user at a particular delivery point is requesting a particular service (such as obtaining signatures from a neighbor).